Based on preliminary data, Dr. Schuett believes that she now has the means to address, in substantial detail, the important issues of regulation of endogeneous Mg turnover, the dynamics of organ-organ Mg exchange and quantification of apparent Mg exchangeable pool size. Thus, it is proposed to investigate the functional relationship between these dynamic measures of Mg metabolism and direct measures of cellular function under three very different physiologic conditions to include: 1) the effect of level of Mg intake, both Mg deficient as well as within the sufficient range; 2) the effect of K deficiency or combined K plus Mg deficiency; and 3) the effect of chronic exercise or exercise plus Mg deficiency. In each experiment she will examine the effect of the specific dietary and/or exercise-induced changes in isotopic measures of Mg metabolism over time. Changes in these parameters will then be related to changes in cellular function as assessed by Na+ K+ ATPase activity and cellular ultrastructure. In addition, she will attempt to identify particular intracellular pools of Mg that may be depleted by conditions that alter soft tissue Mg status. The three experimental conditions she proposed to explore all alter some aspect(s) of whole animal Mg metabolism but have very different effects on extracellular Mg levels. Thus, the proposed combination of studies offers the unique opportunity to test the validity of their isotopic approaches to the assessment of soft tissue Mg status in the face of varying extracellular Mg concentrations.